The use of an aluminum alloy material together with a structural member (steel structural member) using a steel material of a vehicle or the like can reduce the weight of the structural member, thus contributing to reduction in weight of the entire vehicle. The term aluminum alloy material as used herein collectively means pure aluminum and an aluminum alloy, and includes a rolled sheet, an extrusion, a forged material, a cast material, and the like. The term steel material collectively means a steel sheet, a steel die material, a bar steel, and the like.
Steel panel structures placed on a side body of the vehicle are required to have adequate deformation resistance and buckling resistance to side impact and offset front impact, for which standards have become more strict in recent years so as to protect passengers in a vehicle compartment, as well as adequate stiffness and strength which are basic properties. Such steel panel structures are a side sill, a rocker, a pillar, a door, and the like, each being comprised of a panel structure including an outer panel and an inner panel.
In order to ensure impact resistance, that is, to ensure strength for suppressing crush, buckling, or deformation based on new impact standards, it is advantageous to use the aluminum alloy material as a reinforcing member for any steel structural member, such as a panel structure. For example, the aluminum alloy material is interposed as the reinforcing material between the outer panel and the inner panel constituting the steel structural member, which can advantageously improve the deformation resistance and buckling resistance to the side impact and the like.
The reinforcing member of aluminum alloy can exhibit a reinforcing effect by being thicken because of its light weight. In this point, the aluminum alloy reinforcing member is extremely advantageous as compared to a reinforcing member made of a high-tensile material with high strength (high-strength steel sheet). When the same level of reinforcing effect is intended to be achieved by thickening the high-tensile material, a number of (a plurality of) high-tensile materials need to be laminated due to the limited thickness of one high-tensile material itself obtained by thickening. This results in an increase in number of parts, and a great increase in weight. That is, the aluminum alloy reinforcing member can suppress the increase in weight to the minimum level due to addition of the reinforcing member as compared to the steel reinforcing member which can achieve the same level of effect. Furthermore, the aluminum alloy reinforcing member can reduce the number of assembly members without increasing the number of parts.
Thus, for example, the following examples are known and employed as a bonded body of different materials provided by welding and bonding the steel material and aluminum material in the vehicle body:    (1) a door beam (reinforcement of aluminum alloy hollow structure), and a door panel made of steel;    (2) reinforcement by an aluminum alloy hollow structure into a steel panel structural member, such as a steel center pillar or a side sill;    (3) a steel bumper or side member, or a bumper stay or crushable box of an aluminum alloy hollow structure;    (4) alloying an inner panel or outer panel of a steel panel structural member in a large-sized panel, such as a hood or a door, with an aluminum alloy; and    (5) a roof panel of an aluminum alloy plate, a steel side member outer, and a steel roof side rail.
If the aluminum alloy material and the steel material can be spot welded to each other in use of the aluminum alloy material, a spot welding process for use of only the steel material can be executed as it is in assembling a body for the vehicle or the like. This advantage becomes great in a large-sized panel, such as the roof, hood, or door, among the above applications when the spot welding can be performed on a different-material-bonding member (different material bonding panel) with a large different-material bonding area (different-material bonding length) between a panel molded from the aluminum alloy sheet (hereinafter referred to as an aluminum alloy panel or a panel made of aluminum alloy) and a panel molded from the steel sheet (hereinafter referred to as a steel panel or a panel made of steel).
In the case of welding and bonding the steel material and the aluminum material, however, brittle intermetallic compounds tend to be generated in a bonded part, which makes it very difficult to obtain the bonded part (bonding strength) with reliability and high strength. Thus, in the related art, the bonding of the bonded body of different materials is performed by using a bolt, a rivet, an adhesive, or the like. This degrades the efficiency of the above-mentioned assembly process of a vehicle body structure for the vehicle, and thus poses problems including the reliability and cost of a bonding joint, and the like.
In the related art, bonding of the bonded body of different materials is performed together with the bolt, rivet, adhesive, or the like. In many cases, however, the aluminum alloy material may be substituted for one steel material, which is one of steel materials welded and bonded to each other in the related art. Thus, in a manufacturing process of the vehicle body which is established as a production process of welding and bonding, not the welding and bonding method used in the related art, but the above-mentioned different bonding method is employed for the steel material and aluminum material, which remarkably reduces the efficiency of the assembly process of the vehicle body structure.
Accordingly, spot welding methods of such a bonded body of different materials have been hitherto studied well. For example, a spot welding method that can surely bond different materials so as to have high strength without using an expensive cladding material or insert material is disclosed (see Patent Documents 1, 2, and 3). The spot welding method involves superimposing steel sheets and aluminum sheets in the form of multilayer, such as three layers, four layers, or six layers with the aluminum sheet sandwiched between the steel sheets.
In this technique, a superimposed part of the multilayer is held between a pair of electrodes, causing a large current to flow through between the electrodes for a short time thereby to remove a molten part of the aluminum sheet from a spot weld zone. As a result, in a weld bead, the steel sheets are directly bonded to each other without the aluminum sheet, which prevents the generation of intermetallic compounds at a bonding interface. An example of this form is disclosed in which an end edge of the steel sheet is bent back by a hemming process, and at the same time an aluminum sheet is sandwiched between a bent piece and the steel sheet, whereby this three-layered superimposed part is spot welded.
[Patent Document 1]
Japanese Unexamined Patent Publication No. Hei7(1995)-328774
[Patent Document 2]
Japanese Unexamined Patent Publication No. Hei9(1997)-155561
[Patent Document 3]
Japanese Unexamined Patent Publication No. 2003-236673